Director Amma Asante came to my attention with Belle, which smuggled questions of race and racism into an otherwise conventional period piece. With A United Kingdom, Asante again incorporates race into a traditionally English genre that, for the most part, tends to sidestep such issues.

Here, the genre is a classical romance, the kind with a sweeping scope over many years where the central couple’s passion for one another is tested by social disapproval and international events alike. Where Belle inserted a black woman into an upper class, exclusively white society, A United Kingdom finds as its protagonist Seretse Khama. Seretse (David Oyelowo) is from Bechuanaland – Botswana – and resides in 1940s post-war London while completing his studies.

A romance with a London clerk named Ruth Williams (Rosamund Pike) is the focus of the film’s first – and worst – half hour. Oyelow and Pike are warm presences on screen, but there’s a comfortable bond between them rather than the electricity needed. The pedestrian particulars of their courtship, which culminates in a proposal, don’t help matters, especially given that the racism they face feels perfunctory: things we’ve seen in dozens of films before, stripped of their emotional resonance.

That romance, however, is merely a Trojan horse for Asante and screenwriter Guy Hibbard’s true purpose: to interrogate the economic and political underpinning that proliferates racism. That interrogation is enabled by the chosen historical setting; you see, Seretse is not merely a travelling student, but Prince Seretse Khama of Bechuanaland, destined to rule his nation. His marriage to Ruth attracts attention beyond the muted disapproval of Ruth’s parents, as English politicians set about sabotaging their relationship.

A United Kingdom begins with a boxing match, and throughout it remains a battle: following Seretse and Ruth’s fight for approval from the Batswana people, fight for understanding from Seretse’s uncle (Vusi Kunene) and a fight against the British government’s attempts to subvert Seretse’s reign. As the film chronicles the specifics of Seretse’s struggle – his exile from his country, and broken promises from the likes of Winston Churchill – it demonstrates that the disapproval of Seretse and Ruth’s marriage is grounded, like most instances of racism, is more than shallow bigotry.

England is beholden to apartheid-era South Africa, and can’t be seen to support an interracial marriage from a neighbouring country lest its diplomatic bonds be strained or severed. Economic factors are an integral part of this decision making, and the film’s second half derives much of its tension from the impending discovery of diamonds beneath Bechuanaland – a wealth that Seretse justifiably worries will be exploited by England at the expense of his people.

As a depiction of historical racism and its causes, then, A United Kingdom is remarkably nuanced. Unfortunately, the film itself feels somewhat inert. Perhaps this is a simply a reflection of how closely it hews to the handsome period piece model, and the formal blandness that goes hand-in-hand with that approach. (Thankfully, Oyelowo is on hand every 15 minutes or so to deliver a rousing speech.) Or perhaps it’s the way that same nuance isn’t granted to the English villains of the piece; I don’t expect to sympathise with these men, but the screenplay resists any significant insight into their motivations. If you want moral complexity, maybe don’t cast Draco Malfoy as your snivelling ambassadorial assistant.

I think the real reason I felt let down by A United Kingdom, however, was in the way it allows Ruth and Seretse’s romance – and, to a lesser extent, England itself – to overshadow the story of Botswanan independence. It’s not just the disappointing first thirty minutes, but that when Ruth moves to Bechuanaland, the country is too thinly sketched to feel authentic. Asante relies on one or two key characters – Seretse’s uncle, his sister (Terry Pheto) – to stand in for the nation proper, yet examines the intricacies of English politics in a great deal more depth. A United Kingdom’s title presumably refers to Botswana … and, yet, it’s the United Kingdom that receives all the attention.